Opto-iso

[Tillman]

I have an AB plc5 with remote racks which intermittently giving me rack fault. Nothing we've done so far works. We even had a rep from AB to come out to to look over our DH+ network, but he's stumped. At this point I'm entertaining the idea of isolating my discreet I/O field wiring, and wondering if any one has had good experience with a particular brand/type of isolator. Thanks,

 

[roydm]

Are your power supply negatives referenced back to the same common point. I have seen where there was enough difference in voltage to bias off the signals.

Roy

 

[Tillman]

Hi Roy. These remote racks are housed in several cabinets in the field. Each cabinet has a UPS which feeds a 24VDC PS. This PS then feeds the in-chasis AB PS and all I/O power for that cabinet. So, yes, voltage return is common to all.

 

[controlsdude]

any intermintent fault usually points to either bad grounding or the sheild is not terminated correctly on the remote io link, look for frayed wiring. Is the terminating resistor correct and on both ends?

does it happen during operation or all the time?

The worst things that usually happen in panels is the terminals get loose. I would get a maintenance guy to go through the panel and retighten terminals. Make sure the maintenance guy looks for frayed wiring.

 

[Tillman]

Hello Controldude. I personally checked all the terminators and proper termination on the io link and chasis grounding. I even checked each point with a scope, and there was no indication of ground loop, reflected signals, congested or excessive attenuation. The timing of the fault appears random. Sometimes it went weeks without one, sometimes several in a day. I kind of rule out a damaged blue hose because the problem doesn't get worse over time.

 

[controlsdude]

Go to Ab.com and seach for this PDF on remote IO adaptors. Look in the troubleshooting guide chapter 4. Should get you focused on your problem.

Remote IO Adaptor Module User manual
1771-asb

1771-um001_-en-p.pdf

Is there a lot of vibration? Are your cards locked in by the tab on the front of the rack?

intermittent problems are obviously hard to find.

 

[roydm]

I once saw a Modicon PLC that would run fine most of the time but crash for no apparent reason. It turned out that one of the power supplies was jumpered for 240 V but operating on 120. We eventually noticed that it failed whenever a large motor started. I'm sure you have checked all the simple stuff already.
Roy

 

[Tillman]

Appreciate you both for replying.
There's no vibration whatever to the chasis in question. And yes, cards are locked in with the tabs.
I have been using the -asb user manual as a guide.
Roy- that's a good catch on that Modicon. Isn't it typical how a simple installation error would cause so much grief! Just so happened, yesterday we corrected a different dh+ network problem stemed from incorrect setup of a pair of Ecotec fiber to copper converters.
Any thought on what would work well to isolate i/o wiring?

 

[itsmoked]

Sorry but I have seen many failures via flaky comm driver chips. They work mostly and hence they get out thru testing.

Ethernet and RS485,(and most PLC stuff is just RS485 hardware no matter what screwball name they toss on it), both have gone flaky on me MANY times.

I think you may need to change whatever has the actual control hardware. As much time, as you no doubt have spent on this, with no clear solution, or improvement, would point to that now as the most effective next step.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[controlsdude]

When I did have a hand in hardware design and software at the same time. I made sure that the processor and backplane power for the cards was seperate from the io power. Use of shielded transformers was also a plus to cut down on noise from derived 480vac that was alos used for motors in the same control cabinet.


What about this thread, it talks about how to cut down on noise, it for a slc but there would be a similar product or the same for plc5 output cards that could be causing these problems.

"How to check voltage on a 1746-OA16 card"

If you have eliminated the problems in the manual on the asb module then it has to be a noise maker.